This invention relates to compositions comprising polyolefin graft copolymers and fluorinated olefin polymers.
Polymers produced from monomers substituted at the alpha carbon atom such as methacrylates, methacrylonitriles, and xcex1-methylstyrene are thermally unstable and are known to depolymerize to their corresponding monomers at temperatures greater than 230xc2x0 C. Above 300xc2x0 C., poly(methyl methacrylate) (PMMA) depolymerizes rapidly at high conversions ( greater than 95%). Typical extruding and molding temperatures for such polymers are 200xc2x0-290xc2x0 C. Significant depolymerization to the respective monomers would occur in this temperature range, thereby affecting the safety of the operation as well as the properties of the product. Polymethacrylates, and in particular PMMA, are the polymers from alpha-substituted monomers most widely used in commercial applications. In order to broaden the range of applications for these polymers, it is critical to improve their thermal stability.
In the manufacture of graft copolymers comprising a backbone of a propylene polymer material, to which is grafted PMMA, small amounts of non-methacrylate monomers such as methyl acrylate, butyl acrylate and styrene are typically copolymerized with the methyl methacrylate to improve thermal stability, since graft copolymers of methyl methacrylate and one or more of these monomers are much more stable to heat and undergo degradation at relatively higher temperatures. However, addition of these monomers affects the mechanical properties of the graft copolymers as well as the molecular weight and grafting efficiency.
Other methods have been proposed for improving the thermal stability of polymers of monomers substituted at the alpha carbon atom. For example, U.S. Pat. No. 6,046,273 describes a method for improving the thermal stability of graft copolymers of alpha-substituted acrylates by copolymerizing a 1-3 carbon alkyl-substituted acrylic acid with the alpha-substituted acrylate during the preparation of graft copolymers of a propylene polymer material. In addition, WO 00/08078 describes a method for further improving the thermal stability of acrylic grafted copolymers by polymerizing acrylic monomers onto a backbone of a propylene polymer material in the presence of a 4-vinyl-substituted 5-12 C cyclic 1-alkene.
Fluorinated polymers are characterized by resistance to harsh chemicals as well as stability toward heat, ultraviolet light, high energy radiation, and oxidation. Polyvinylidene fluoride, a member of the class of fluorinated polymers, is a semi-crystalline material with a high dielectric constant that can easily be processed on conventional molding and extrusion equipment. It also has high mechanical and impact strength, and high resistance to creep, fatigue and abrasion. Film made from this polymer has excellent oxygen and moisture barrier properties.
Fluorinated polymers have been blended with other polymers to improve properties. For example, U.S. Pat. No. 5,034,460 describes a composition comprising a vinylidene fluoride-based polymer, a graft copolymer formed from the graft polymerization of an alkyl methacrylate onto a fluorine-containing polymer, and at least one organic solvent. The resulting composition is reported to provide coatings having weather resistance, chemical stability, surface lubricity, and enhanced adhesion to substrates. Similarly, U.S. Pat. No. 5,188,873 describes a composition based on polyolefins containing at least one fluoropolymer and at least one low-density polyethylene, which can be employed for production of articles with improved optical properties when compared to compositions not containing a fluoropolymer.
Thus there is still a need for a method to increase the thermal stability of graft copolymers that include polymerized acrylic monomers substituted at the alpha-carbon atom while maintaining the physical properties of the graft copolymers.
The compositions of this invention comprise
(1) a graft copolymer comprising a backbone of a propylene polymer material having graft polymerized thereto a monomer selected from the group consisting of (a) an ester of an acrylic acid substituted at the alpha carbon atom by a 1-3 alkyl group and (b) a combination of (i) an ester of an acrylic acid substituted at the alpha carbon atom by a 1-3 alkyl group and (ii) an ester of an unsubstituted acrylic acid, or an acrylic acid substituted at the alpha carbon atom by a 1-3 carbon alkyl group, and
(2) about 1 weight % to about 25 weight % of a fluorinated olefin polymer, wherein the ratio of the polymerized monomer to the fluorinated olefin polymer is about 25:1 to about 0.5:1.
Also disclosed is a process for improving the thermal stability of a graft copolymer comprising blending
(1) a graft copolymer comprising a backbone of a propylene polymer material having graft polymerized thereto a monomer selected from the group consisting of (a) an ester of an acrylic acid substituted at the alpha carbon atom by a 1-3 C alkyl group and (b) a combination of (i) an ester of an acrylic acid substituted at the alpha carbon atom by a 1-3 alkyl group and (ii) an ester of an unsubstituted acrylic acid, or an acrylic acid substituted at the alpha carbon atom by a 1-3 carbon alkyl group and
(2) about 1 weight % to about 25 weight % of a fluorinated olefin polymer, wherein the ratio of the polymerized monomer to the fluorinated olefin polymer is about 25:1 to about 0.5:1.